Method for producing a motor vehicle component and motor vehicle component

ABSTRACT

A method for producing a motor vehicle component having two regions, with different strength is disclosed. A first one of the two regions has a high strength and a second one of the two regions has a higher ductility and lower strength relative to the first region. In a motor vehicle component produced with the method, the first region has a strength between 1400 and 1600 MPa at a breaking elongation A5&gt;13% and the second region has a tensile strength between 950 and 1050 MPa at a breaking elongation A5&gt;16%.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2011 057 007.1, filed Dec. 23, 2011, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a motor vehicle component and motor vehicle component.

The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.

For producing motor vehicle components, the hot forming and press hardening technology has become established in recent years with which it is possible to produce motor vehicle components from hardenable steel alloys which have particularly high strength at a low own weight. Hot formed and press hardened components are used in particular as crash relevant components or as motor vehicle structural components. A sheet metal blank is heated to above austenization temperature, inserted into a hot forming tool where it is hot formed, and subsequently press hardened. During this, a martensitic structure is generated which has high strength values but small breaking elongation values, for example smaller than 7%. The hot formed and press hardened components do not only have advantages but due to their hard and partly brittle properties also certain disadvantages. In coupling regions or in the regions of recesses, a tearing or tearing off of the component can occur for example in an accident or crash of a motor vehicle.

It would therefore be desirable and advantageous to provide an improved method for the production of a motor vehicle component with which it is possible to establish better strength properties in the motor vehicle component compared to methods known from the state of the art and at the same time high ductility at least in regions of the motor vehicle component.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for producing a motor vehicle component having two regions with different strength properties, includes the steps of providing a blank made from hardenable steel sheet, heating the blank to at least 800 to 1100° C. transferring the heated blank into a furnace with two temperature zones, so that a first region of the blank is arranged in a first one of the two temperature zones and a second region of the blank is arranged in a second one of the two temperature zones, wherein the first zone has a first temperature essentially corresponding to an austenization temperature, and the second zone has a temperature between 300 and 600° C., cooling the second region of the blank to a second temperature between 300 and 450° C., holding the first region at the first temperature and the second region at the second temperature for a defined period of time, and hot forming and press hardening the blank into the motor vehicle component, thereby establishing a tensile strength between 900 and 1300 MPa and a breaking elongation A5 of more than 16% in the second region.

Within the framework of the invention, a blank made of hardenable steel sheet is thus provided and heated to at least austenization temperature. Within the context of the invention, austenization temperature relates to the austenization temperature of the respectively used steel material. Depending on the alloy composition of the used steel material, the austenization temperature is at least 800° C. Preferably, the austenization temperature in a steel alloy used according to the invention is a heating temperature between 800 and 1100° C., preferably between 900 and 1000° C. The heating can occur in a heat station, for example by conductive or inductive heating. Heating by heat radiation, convection or by infrared or other heating methods is also possible.

After heating to at least austenization temperature, the thus heated blank is transferred to a furnace. The furnace can be a continuous furnace or in the case of heating to austenization temperature and the subsequent furnace, a furnace system which is connected with a conveyor belt.

In the furnace, the heated blank is held at a temperature, wherein the furnace has two temperature zones, and wherein the temperature zones have different temperatures. A first zone has a temperature which essentially corresponds to the austenization temperature or has a temperature which is at least above 700° C., preferably above 800° C. and in particular between 900 and 1000° C.

According to another advantageous feature of the present invention, the blank can be heated to austenization temperature +/−100° C., thereby enabling to generate an austenization which is essentially 100% complete, however at the same time providing a partial austenization. At temperatures below the austenization temperature, the blank austenizes only partially, whereas at temperatures above the austenization temperature the blank austenizes completely.

The second zone has a lower temperature compared to the first zone, preferably a temperature which is between 300 and 600° C. The blank is held in the furnace with the two different temperatures for a holding time at the respective temperatures in the zones and at the temperatures which are established in the blank itself as a result thereof.

The second region is held in the second zone of the furnace at a temperature of the material of the blank, i.e., a temperature of the second region between 300 and 450° C. Prior to this, it is necessary however, to cool the second region to the temperature between 300 and 450° C. For the cooling for example air nozzles or similar devices can be used. An active cooling of the second region also occurs shortly before or during the transfer into the furnace, for holding the temperature or directly at the beginning of the holding of the temperatures in the furnace. The first region is arranged in the first zone of the furnace and also held there for a certain period of time.

Subsequently, the blank which is held at two different temperatures is transferred into a hot forming tool, hot formed there and press hardened, so that a hot formed and press hardened motor vehicle component with two different strength regions is created. The first region has a high strength, whereas the second region has a ductility which is higher compared to the first region. Between the first region and the second region, a transitional region is preferably formed, wherein, the transitional region is between 20 and 110 mm, preferably between 30 and 100 mm and in particular essentially 50 mm.

By this, in particular motor vehicle components for example motor vehicle columns, sills or roof pillars can be produced within the framework of the invention, which have a higher ductility and a higher strength, and are in particular connection regions. In case of a motor vehicle accident, no premature crack formation occurs and the component therefore does not tear off, but rather the connection of the individual components is held for a long time so as to conduct crash energy in a targeted manner or to absorb crash energy by deformation. It is also possible to produce armoring components with the method according to the invention which have then a high resistance against being fired at or against bombardment, also against projectile weapons or detonations. It is also possible within the framework of the invention to produce crash relevant components, for example cross beams or crash boxes with the method.

Particularly preferably, the two regions i.e., the regions with a higher ductility relative to the first regions are produced at the motor vehicle components which serve as deformation regions, passage openings, coupling points or other connecting regions. Within the framework of the invention, it is thus possible to produce a component with multiple first and multiple second regions. It is also possible within the framework of the invention, to produce a component which has a first region and two, three or more second regions.

According to another advantageous feature of the present invention, a hardenable steel is used for performing the method according to the invention, which has an alloy in which the following alloy components are used:

Carbon (C)  0.14-0.3% Manganese (Mn)   0.8-2.5% Silicon (Si)   1.5-2.5% Chromium (Cr) max. 0.4% Aluminum (Al) max. 0.1% Nickel (Ni) max. 0.3% Boron (B) 0.0008-0.005% Titanium (Ti)  0.005-0.1% Niobium (Nb) max 0.1%, Remainder iron and smelting related contamination.

The contaminations can be on one hand the smelting related contaminations during manufacture of the raw components or semi-finished products, these can however, also be contaminations which occur or are generated during hot forming and press hardening or during heat treatment.

In such an alloy, a transformation of the structure to austenite first occurs by heating to above austenization temperature, i.e. a temperature above AC3 point of the alloy. As a result of the intermediate treatment between the heating and the insertion into the hot forming tool, an essentially austenitic structure is retained in the first region whereas in the second region a bainitic structure, in particular a bainitic structure in the lower bainitic region is established. A high silicon content causes formation of silicon oxide at a surface, which prevents premature scaling. This measure enables decreasing a scaling or even omitting protective gas during the heating. This saves energy costs.

After holding the temperature, i.e., after the intermediate heat treatment, a hot forming with subsequent press hardening is performed. The blank is cooled to a temperature which is preferably below 250° C., in particular below 200° C. and especially preferably below 150° C., and press hardened, so that a martensitic structure is established in the first region and a bainitic structure, in particular a bainitic structure in the lower bainitic region is established in the second region.

Due to the special composition of the steel, in particular the relatively large amount of added silicon, not only martensite is generated during hardening. A portion of the austenite is also retained as residual austeninte, which remains stable up to temperatures of −100° C. The silicon in the steel further prevents carbide formation, due to which carbon is available for stabilization of the residual austenite. The residual austenite provides the motor vehicle component, which is produced according to the method according to the invention, with a higher breaking elongation also in the regions of the first kind than is the case in classical boron alloyed, purely martensitic hot forming steels, for example 22 MnB5.

According to another advantageous feature of the present invention, after heating the second region to above austenitic temperature and shortly before or during transfer into the furnace for holding the two different temperatures, the second region is actively cooled, in particular with a cooling rate between 100 Kelvin per second and 10 Kelvin per second. The second region is cooled so that the temperature of the blank in the second region is between 300 and 450° C. The cooling rate between 100 Kelvin per second and 10 Kelvin per second is to be selected in dependence on the strength values to be established in the second region. Thus, it is possible at a lower cooling rate which is in the range of 20 Kelvin per second, to cool directly into the bainitic phase. At a high cooling rate which is in the range of between 80 and 90 Kelvin per second depending on the cooling temperature it is initially cooled below the martensite starting temperature, i.e., the temperature at which the austenite starts to transition into the martensitic state, and transition into the bainitic structure occurs during subsequent holding of the temperature in the furnace. This also allows establishing a bainitic martensitic mixed structure.

Overall, by holding the second region at a temperature between 300 and 450° C. and the subsequent hot forming and press hardening, a bainitic structure is established in the second region, which is located in the lower bainitic region of a time temperature transformation diagram. At the same time, due to the holding at essentially austenization temperature, an essentially martensitic structure is established in the first region.

Preferably, the blank is held in the furnace for a time between 5 and 400 seconds at the two different temperature ranges. The holding time largely depends on the strength and ductility values to be established in the second region because due to the holding time, in particular in connection with the holding temperature of the second region, the establishment of the bainitic structure or a martensitic bainitic mixed structure is achieved. The previously described method steps according to the invention and the use of the alloy according to the invention enable establishing a tensile strength between 1200 and 1700 MPa in the first region, preferably between 1300 and 1600 MPa and preferably between 1450 and 1550 MPa, at a breaking elongation A5 greater than 13%. The breaking elongation is to be limited within the framework of the invention at an upper limit of about 40%, preferably 30% and especially preferably 20%, in particular for the first but also for the second region.

In the second region, a tensile strength between 900 and 1300 MPa preferably between 950 and 1050 MPa is established at a breaking elongation from A5 of more than 16%, in particular of more than 17%. For example, at a furnace temperature in the second zone of about 350° C., a tensile strength in the second region of 1000 MPa, a yield point Rp 0.2 of about 650 to 700 MPa at a breaking elongation A5 of more than 16% is established. At a furnace temperature of 600° C. it is possible to establish a tensile strength of 800 MPa, a yield point Rp 0.2 of about 600 MPa at a breaking elongation A5 greater than 17%.

According to another advantageous feature of the present invention, a yield point Rp 0.2 between 550 and 800 MPa, in particular between 600 and 700 MPa can be established in the second region by the previously mentioned method steps when using the above mentioned alloy according to the invention.

By using the above mentioned alloy with the high silicon content according to the invention, the surface of the component scales less during heating than the conventional hot forming steels. This enables producing a hot formed, press hardened component with a surface which can be directly further processed without prior jetting. Thus, for example welding or bonding work can be carried out or a coating, varnishing or a KTL coating can be performed. Also, the produced motor vehicle component is tempering resistant. It is thus possible to galvanize the motor vehicle component according to the invention even at temperatures up to 400 or even 450° C., while at the same time retaining the previously mentioned strength properties.

According to another advantageous feature of the present invention, the method according to the invention can be performed on a continuous furnace. Here, only the holding of the temperature for a defined period of time on the continuous furnace may be performed or the entire heating process until transfer of the heated and heat treated blank into a hot forming tool. It is thus possible to first heat the blank to above austenizing temperature in a heating system for example via induction, infrared, hot air or the like. Subsequently, the blank is transferred from the heating system into the continuous furnace.

In the continuous furnace, two different temperature zones can be provided so that in a first zone, a first region of the blank is held at essentially austenization temperature and in a second zone of the continuous furnace, the second region of the blank is held at a temperature between 300 and 450° C.

According to another advantageous feature of the present invention, it is also possible however, to provide a complete continuous furnace system, wherein at the beginning of the continuous furnace the entire blank is heated in a first section of the continuous furnace to above austenizing temperature and then in a second section it is held at two different temperature zones. Within the framework of the invention, it is then necessary however, to provide active cooling means between the first and second section for cooling the second region of the blank from above austenizing temperature. The cooling means can for example include air cooling or a liquid cooling. For example, air nozzles can be provided which enable a corresponding cooling within the previously mentioned cooling rates. It is also possible to perform the cooling via cooling plates, i.e., conductively.

According to another advantageous feature of the present invention, it is also possible to use a continuous furnace with three temperature zones. In this case, the austenization temperature is then essentially held in the first region in a first zone, wherein the second zone is divided into two subzones, wherein in a first subzone, a first subzone temperature is established and in a second subzone, a second subzone temperature is established.

According to another aspect of the present invention, a motor vehicle component with two different strength regions is produced according to a method according to at least one of the previously mentioned features and is characterized in that a first region has a strength between 1400 and 1600 MPa and a breaking elongation AS greater than 13%, and a second region has a tensile strength between 950 and 1050 MPa and a yield point Rp 0.2 between 600 and 700 MPa at a breaking elongation A5 greater than 16%.

In the motor vehicle component according to the invention, the first region has essentially a martensitic structure, which can also contain residual austenitic portions, and the second region has an essentially bainitic structure, in particular a bainitic structure which is produced by quenching from the bainitic region of a ZTU diagram. The bainitic structure can also have residual martensite portions.

Such a motor vehicle component is particularly useful as component which is required to have a high strength and with this a high resistance against mechanical stresses, however, at the same time has a ductility in one or more connecting regions, i.e., regions of the second type, which prevents a tearing off or tearing away from the vehicle body. For example, a motor vehicle column is produced in this way which has a higher ductility in a connecting region for a roof pillar and a connecting area for a sill, i.e., it has second regions, however the region between the roof space and the sill is a first region which has a higher strength.

The previously mentioned features can be combined in any desired manner within the framework of the invention with the associated advantages, without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 shows a heat treatment system with a heat station and a two zone furnace,

FIG. 2 shows a continuous furnace with two different temperature zones,

FIG. 3 shows a continuous furnace with three different zones and

FIG. 4 shows a ZTU diagram for producing the second regions

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is shown a heat treatment system 1 with a heating station 2 in which the blank 3 is heated to above austenization temperature T1. For this, an austenization temperature T1 of preferably between 900 and 1000° C. is present in the heating station 2. The heating station 2 can for example be a pizza oven, a conductive, inductive or other heating station 2. Subsequent to this, the blank 3 is transferred into the furnace 4, wherein the furnace 4 has two different zones with different temperatures. For this, a first zone 5 has a temperature T2, which essentially corresponds to the austenization temperature T1. In the first zone 5, a first region 7 of the blank 3 is essentially held at or above austenization temperature. In the second zone 6, a temperature of T3 is present which essentially is between 300 and 650° C. and a second region 8 of the blank 3 is essentially held at a temperature T3 between 350° C. and 450° C. The furnace 4 can already be a continuous furnace 4 so that the component can afterwards be transferred into a hot forming and press hardening tool 9 and is hot formed and press hardened in the same, in particular quench hardened.

FIG. 2 shows a two-zone continuous furnace 4, wherein the blank 3 is first heated in a first section 10 to above austenization temperature T1 and subsequently transported in transport direction 11 through the continuous furnace 4.

In a second section 12 which is downstream of the first section 10, two different temperature zones are formed, wherein a first zone 5 has the temperature T2 and a second zone 6 has the temperature T3. Between the first and the second sections (10, 12) the second region 8 of the blank 3 is cooled with here not further shown cooling means. Subsequent to this, the such treated blank 3 is again transferred into a hot forming and press hardening tool 9 and formed into the motor vehicle component according to the invention.

FIG. 3 shows a continuous furnace 4 with three different temperature zones, wherein the continuous furnace 4 has a first section 10 which analogously corresponds to the continuous furnace 4 shown in FIG. 2. Also in this case, the blank 3 is heated to above austenization temperature T1 and subsequently transported in transport direction 11 into the second section 12. In the second section 12, a first zone 5 is formed which has a temperature T2 which essentially corresponds to the austenization temperature T1.

The second zone is divided into a first sub zone with a temperature T4 which essentially is between 250 and 450° C. and a second sub zone 14 with a temperature T5 which essentially is between 400 and 600° C., preferably between 450 and 550° C. Again a first region 7 is essentially held at above austeniztation temperature T1, whereas the second region 8 is subjected to a two step heat treatment by the first subzone 13 and subsequently by the second sub zone 14. Subsequent to the continuous furnace 14 according to FIG. 3 the heat treated blank 3 is again transferred into a hot forming and press hardening tool 9 and there hot formed and press hardened to the motor vehicle component.

FIG. 4 shows a ZTU temperature diagram for the alloy composition mentioned in the context of the present invention, wherein the ordinate shows the temperature in degree Celsius and on the abscissa the logarithmic time t in seconds. The ZTU-diagram of FIG. 4 shows the time temperature transformation for the second region in an interval between the first curve 15 with low cooling rate and a second curve 16 with higher cooling rate. It can be seen that according to the first curve, proceeding from a temperature which is in the range of the austenization temperature T1, the second region is cooled into a lower bainitic structure 17, and in the case of the second curve 16 is initially cooled slightly below the martensite start temperature MS and is then held at this temperature for a period of time. In the case of the first curve 15, the second region in the lower bainitic structure is also held for a period of time, and in the two curves 16, 17 is then quenched out of the bainitic structure. In the case of the second curve 16, it is possible to establish a martensitic bainitic mixed structure. Within the framework of the invention, it is possible to select all cooling rates and holding times for establishing an essentially bainitic structure for the second region, which structure is located in the lower bainitic structural region 17.

While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:
 1. A method for producing a motor vehicle component having two regions with different strength properties, comprising the steps of: providing a blank made from hardenable steel sheet; heating the blank to at least 800 to 1100° C.; transferring the heated blank into a furnace with two temperature zones, so that a first region of the blank is arranged in a first one of the two temperature zones and a second region of the blank is arranged in a second one of the two temperature zones, wherein the first zone has a first temperature essentially corresponding to an austenization temperature, and the second zone has a temperature between 300 and 600° C.; cooling the second region of the blank to a second temperature between 300 and 450° C.; holding the first region at the first temperature and the second region at the second temperature for a defined period of time; and hot forming and press hardening the blank into the motor vehicle component, thereby establishing a tensile strength between 900 and 1300 MPa and a breaking elongation A5 of more than 16% in the second region.
 2. The method of claim 1, wherein the blank is heated in the heating step to 900 to 1000° C.
 3. The method of claim 1, wherein the blank is heated in the heating step to austenization temperature +/−100° C.
 4. The method of claim 1, wherein the hardenable steel is an alloy comprising the following alloy components in weight percent: Carbon (C)  0.14-0.3% Manganese (Mn)   0.8-2.5% Silicon (Si)   1.5-2.5% Chromium (Cr) max. 0.4% Aluminum (Al) max. 0.1% Nickel (Ni) max. 0.3% Boron (B) 0.0008-0.005% Titanium (Ti)  0.005-0.1% Niobium (Nb) max. 0.1%,

Remainder Iron and smelting related contaminations.
 5. The method of claim 3, wherein in the cooling step the second region is cooled from the austenization temperature to between 300 and 450° C. at a cooling rate of between 100K/s and 10K/s.
 4. The method of claim 1, wherein the hot forming and press hardening step establishes a bainitic structure in the second region and an essentially martensitic structure in the first region.
 5. The method of claim 4, wherein the bainitic structure is a lower bainitic structure.
 6. The method of claim 1, wherein the period of time is between 5 and 400 seconds.
 7. The method of claim 1, wherein in the first region in the hot formed and press hardened plank has a tensile strength between 1200 and 1700 MPa and a breaking elongation A5 greater than 13%.
 8. The method of claim 7, wherein the tensile strength is between 1300 and 1600 MPa.
 9. The method of claim 7, wherein the tensile strength is between 1450 and 1550 MPa.
 10. The method of claim 1, wherein the second region in the hot formed and press hardened plank has a tensile strength between 950 and 1050 MPa and a breaking elongation A5 of more than 17%.
 11. The method of claim 1, wherein the second region in the hot formed and press hardened plank has a yield point Rp0.2 between 550 and 800 MPa.
 12. The method of claim 11, wherein the yield point is between 600 and 700 MPa.
 13. The method of claim 1, wherein the heating step and the holding step are performed in a continuous furnace system or in a heating station with downstream furnace.
 14. The method of claim 13, wherein the downstream furnace is constructed as continuous furnace.
 15. The method of claim 14, wherein the continuous furnace comprises three temperature zones.
 16. A motor vehicle component produced according to the method of claim 1 and comprising first and second regions, said first and second regions differing in strength, wherein the first region has a strength between 1400 and 1600 MPa and a breaking elongation A5 of greater than 13% and the second region has a tensile strength between 950 and 1050 MPa, a yield point Rp0.2 of between 600 and 700 MPa and a breaking elongation A5 of greater than 16%.
 17. The motor vehicle of claim 16, wherein the first region has a martensitic structure and the second region has a bainitic structure.
 18. The motor vehicle of claim 17, wherein the bainitic structure is a lower bainitic structure. 